1. Field of the Invention
The present invention relates to a cluster printing system for performing printing by using a plurality of electrophotographic recording apparatuses such as printers, facsimile machines or copying machines each capable of manifesting an image by using colored particles such as toner. Particularly, it relates to an image quality control method in an imaging and fixing process having electrification, exposure, development, transfer and fixation for forming a toner image on surfaces of a photoconductor and a sheet of recording paper, recording apparatuses using the image quality control method, and a method for operating the recording apparatuses.
2. Background Art
A conventional recording apparatus using electrophotography has an imaging process for manifesting an image of colored particles on a surface of a recording medium, and a fixing process for fixing the manifested image of colored particles on the recording medium. In this specification, a combination of the imaging process and the fixing process is referred to as imaging engine. Powder called xe2x80x9ctonerxe2x80x9d exclusively used for electrophotography is used as the colored particles. In an electrifying step, the whole surface of a photoconductor is once electrically charged. Then, in an exposure step, the photoconductor irradiated with light is partially electrically discharged. On this occasion, potential contrast between a charged region and a discharged region is formed in the surface of the photoconductor. The potential contrast is referred to as xe2x80x9celectrostatic latent imagexe2x80x9d.
In the next developing step, first, toner particles which are colored particles are electrically charged. As methods for electrically charging toner, there are a two-component developing method using carrier beads and a one-component developing method for electrically charging toner on the basis of friction between the toner and a member or the like. On the other hand, a method called xe2x80x9cbias developmentxe2x80x9d is often used as a method for manifesting the electrostatic latent image.
In the bias development, a bias voltage is applied to a developing roller so that electrostatically charged toner particles are separated from a developing agent on a surface of the developing roller and moved to the surface of the photoconductor by the action of electric field generated between latent image potential generated on a surface of a photoconductor and the potential of the developing roller to thereby form an image. Either the electrostatic charge potential or discharge potential may be used as the latent image potential, that is, as the potential of the image-forming portion of the photoconductor. Generally, the method using electrostatic charge potential as the latent image potential is referred to as xe2x80x9cnormal developing methodxe2x80x9d whereas the method using discharge potential as the latent image potential is referred to as xe2x80x9creversal developing methodxe2x80x9d.
Potential which is either of the electrostatic charge potential and the discharge potential but is not used as the latent image potential is referred to as xe2x80x9cbackground potentialxe2x80x9d. The bias voltage of the developing roller is set to have potential middle between the electrostatic charge potential and the discharge potential. Similarly, the difference between the middle potential (bias voltage) and the latent image potential is referred to as xe2x80x9cdeveloping potential differencexe2x80x9d. The difference between the middle potential (bias voltage) and the background potential is referred to as xe2x80x9cbackground potential differencexe2x80x9d. Generally, the developing potential difference having an influence on developing performance itself is set to be larger than the background potential difference. It is a matter of course that if the developing potential difference is large, developing performance becomes high because generated electric field (referred to as xe2x80x9cdeveloping electric fieldxe2x80x9d) becomes intensive.
On the other hand, the background potential difference has an influence on the image quality of a background portion of an image. If the background potential difference is small, fogging of the background portion increases. If the background potential difference is too large, a rear end portion of the image in a direction of rotation of the developing roller is apt to be chipped. The direction of relative movement of the developing roller and the direction of relative movement of the photoconductor may be equal to each other or may be different from each other.
A plurality of developing rollers may be used in one developing device. A developing device having a plurality of developing rollers rotating in one direction may be provided or a developing device having a plurality of developing rollers rotating indifferent directions may be provided. In this case, there is also known a developing device in which the directions of rotation of adjacent developing rollers are made different to move the two developing rollers from their opposite positions toward the photoconductor so that the developing agent is carried toward the photoconductor while branching from the opposite positions of the developing rollers as if the developing agent was a fountain. The developing device is referred to as xe2x80x9cfountain type developing devicexe2x80x9d. The formation of an electrostatic latent image and a toner image on a surface of the photoconductor has been described above.
Next, variation of the electrostatic latent image on the surface of the photoconductor with time will be described. When the photoconductor deteriorates as printing increases in quantity, the potential of an electrostatic charge region (charge potential) is so lowered that the electrostatic charge region can be hardly charged while the potential of a discharge region (discharge potential) is so heighten that the discharge region can be hardly discharged. The lowering of the discharging capacity is significant in the case where an intermediate potential region is provided so that the intermediate potential region is not perfectly discharged because a sufficient quantity of light is not given at exposure.
The intermediate potential region described here is often used for preventing thickening of an image region such as a thin line region or a halftone dot region in which the edge effect of electric field is so intensive that toner is developed excessively. The variation in potential operates to reduce developing electric field because it reduces the developing potential difference. On the other hand, in addition to this characteristic, the thickness of the photosensitive layer of the photoconductor is reduced by abrasion as printing increases in quantity. The reduction of the film thickness operates to increase the developing electric field. Which of the two antithetical tendencies is predominant varies in accordance with the printing apparatus.
That is, though image quality varies in accordance with variation with time in developing capacity, how the image quality varies depends on the printing apparatus. Reduction of variation in the developing electric field is required for keeping image quality constant with time. For this reason, it is necessary to consider variation in potential and electric field on the surface of the photoconductor.
There is known a method in which the potential on the surface of the photoconductor is detected by a potential sensor and the film thickness of the photoconductor is detected by some method to control the potential on the surface of the photoconductor to keep the developing electric field constant. For example, the related art concerning a method of controlling the surface potential of the photoconductor in consideration of the influence of the electric field has been described in JP-A-11-15214.
Variation in charge density of toner in the developing device is a main cause of variation in image quality as well as variation with time in potential and electric field of the electrostatic latent image on the surface of the photoconductor is a main cause thereof. Hence, there is also known a method for keeping image quality stable by using feedback control to adjust the developing bias voltage on the basis of the detected value of toner mass deposited on the photoconductor. For example, the related art concerning a method of controlling deposited toner mass stably has been described in JP-A-4-146459.
As described above, image quality control (hereinafter referred to as xe2x80x9cimage quality stabilizing controlxe2x80x9d) in the related art is made for keeping image quality constant with time in one recording apparatus but there is no consideration about image quality difference between recording apparatuses in the case where, for example, two or more recording apparatuses are used for outputting continuous printed matter.
If two or more printing apparatuses are used for obtaining one continuous printed matter, there arises a problem that image quality varies discontinuously in different pages printed by the recording apparatuses. The term xe2x80x9ccontinuous printed matterxe2x80x9d used here in means printed matter such as a booklet having different sheets of recording paper but having relevant contents in front and rear pages and recognized as one object by a user requiring information written in the printed matter. The continuous printed matter is referred to as xe2x80x9cjobxe2x80x9d in this specification. Printing of one job by two or more recording apparatuses is referred to as xe2x80x9ccluster printingxe2x80x9d in this specification. Incidentally, one recording apparatus in this specification is constituted by one imaging engine. For example, two imaging engines may be connected to each other and put as one apparatus into a casing. Even in this case, the two imaging engines are regarded as two recording apparatuses persistently in this specification.
An object of the invention is to provide an electrophotographic printing system in which image quality in one job is prevented from varying discontinuously even in the case where cluster printing is made.
In order to suppress image quality difference between a plurality of electrophotographic recording apparatuses, in accordance with the invention, image quality stabilizing control is applied to each of the electrophotographic recording apparatuses in such a manner that a certain electrophographic recording apparatus is used so that image quality of the other electrophotographic recording apparatuses is controlled on the basis of detected information of the certain electrophotographic recording apparatus.